THE MOLARS. 629 



nence on the third molar to the corresponding part of the opposite tooth, 

 0.108 m. 



3d. Finally, behind : from the middle of the posterior border of the sixth 

 molar to the corresponding part of the opposite tooth, 0.097 m. 



Direction. All the superior molars have not the same direction. (See 

 Fig. 270.) If the head be placed horizontally, the first is almost vertical, while 

 the others are curved from below upward and from before to behind in a degree 

 which varies somewhat according to the subjects. However it may be, this direc- 

 tion is always more pronounced in the last than in those which precede it. 



Relations. The superior molars are implanted in their alveoli of a quad- 

 rangular form, moulded exactly upon the former, and separated from each other 

 by thin osseous septa, whose thickness augments from below upward. The bottoms 

 of the last three alveoli alone project into the maxillary sinus ; ' those of the first 

 three are, on the contrary, placed in the thickness of the bone. It is for the above 

 reasons that diseases of these teeth sometimes extend to the mucous membrane 

 which lines these sinuses ; that the face, upon its lateral planes, appears convex 

 in young horses and concave in old, whose molars are much shorter; finally, 

 that perforation of the superficial table of the superior maxillary bone is pro- 

 duced in the young by the roots of one or other of the first three molars. 



Length. The following table shows the data which we have obtained in 

 this respect from the head of a horse over six years old, whose teeth have been 

 measured from the middle of their external border to the notch which separates 

 the two roots on this side, roots that are sensibly of the same dimensions : 



1st 0.054m. 



2d . . 0.068m. 



3d ..'....'.. 0.079m. 



4th 0.065m. 



5th . . 0.072m. 



plus the antero-external root, = 0.018 m. = 0.072 m. 



" " " = 0.012 m. = 0.080 m. 



" = 0.008 m. = 0.087 m. 



= 0.014m. = 0.079m. 



-0.008m. = 0080m. 



6th, which presents no roots, has a total length of 0.072 m. 



It might almost be said, from the results of these measurements, that the 

 length of the superior molars augments gradually from the first to the third, and 

 then diminishes from the latter to the sixth. This conclusion is, nevertheless, 

 not altogether accurate, for, as the table will show, the fifth is slightly longer 

 than the fourth. 



b. Inferior Jaw. 



In order to avoid repetitions, the descriptions of the inferior molars will be 

 as concise as possible. These teeth, as we already know, of the same number 

 as in the superior jaw, are also designated by numbers. 



They have the same general form, but are less voluminous and, particularly, 

 less thick in their transverse diameter. 



Faces. The anterior and posterior faces, almost plane, present nothing 

 particular. The anterior face of the first and the posterior face of the sixth are 

 replaced by a slightly-thickened border. 



The external face, E (Fig. 299), is traversed by a longitudinal groove in the 

 first five ; the sixth has two grooves on this face. All are limited in front and 

 behind by a more or less rounded ridge. 



1 In old horses we have seen the inferior maxillary sinus extend beyond the root of the 

 second molar. 



